Trench power semiconductor devices, such as trench power MOSFETs (metal oxide semiconductor field effect transistors), include a p-n junction between a drift region and a body region. On-state resistance (i.e., RDSon) and breakdown voltage are two important parameters in a trench power MOSFET. The on-state resistance of a trench power MOSFET is the drain-to-source resistance of the trench power MOSFET in an on-state, which depends partially on the doping level of the drift region. The breakdown voltage of a trench power MOSFET is the voltage at which a reverse biased body-drift p-n junction breaks down and significant current starts to flow between a source and a drain of the trench power MOSFET by an avalanche multiplication process. It is desirable for a trench power semiconductor device to have a reduced the RDSon in its on-state, and be able to withstand a high drain-to-source voltage during its off-state (i.e., a high reverse voltage blocking capability). However, there is a trade-off in the design of a trench power semiconductor device, between its reverse voltage blocking capability and its on-state resistance.
One technique for improving the reverse voltage blocking capability of a trench power MOSFET involves placing field plates in the drift region, where the field plates are electrically connected to a fixed electrical potential, such as a gate or source potential in the trench power MOSFET, to allow a depletion region expand in the drift region. However, this may result in a high voltage difference between the field plate and those regions of the drift region close to the drain region in the trench power MOSFET, so that a thick field plate dielectric would be required. For example, since the gate and source are typically at the same potential (e.g., 0 volts) during reverse bias, a relatively thick dielectric is required in the trench to withstand the full drain bias. A thick dielectric, however, adversely increases the on-state resistance of the trench power MOSFET. Another technique involves forming one or more field plates in the same trench as a gate electrode, where the trench extends from the body region into the drift region of the trench power MOSFET. This technique, however, not only requires the field plates be directly aligned with the gate electrode in the trench, but is also difficult to implement and expensive.
One technique for reducing the on-state resistance of a trench power semiconductor device involves reducing the device cell pitch and the pitch of the field plates. However, this cannot be achieved without the expense of reducing the field plate dielectric thickness, which would adversely affect the reverse voltage blocking capability of the trench power semiconductor device.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a power semiconductor device, such as a power MOSFET, with a reduced on-state resistance without compromising the reverse voltage blocking capability of the power semiconductor device.